A display device using a liquid crystal compound (in the application, a term “liquid crystal compound” is used as a generic term for a compound having a liquid crystal phase and a compound having no liquid crystal phase but being useful as a constituent of a liquid crystal composition) has been widely used for a display for a watch, a calculator, a word processor and so forth. The display devices utilize an optical anisotropy, a dielectric anisotropy and so forth of the liquid crystal compound.
In a liquid crystal display device, a classification based on an operating mode for liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA) and polymer sustained alignment (PSA). A classification based on a driving mode of a device includes passive matrix (PM) and active matrix (AM). The passive matrix (PM) is classified into static, multiplex and so forth, and the AM is classified into thin film transistor (TFT), metal insulator metal (MIM) and so forth.
The liquid crystal display devices include a liquid crystal composition having suitable physical properties. In order to improve characteristics of the liquid crystal display device, the liquid crystal composition is preferred to have suitable physical properties. General physical properties necessary for the liquid crystal compound being a component of the liquid crystal composition are as follows:
(1) being chemically stable and physically stable;
(2) having a high clearing point (a phase transition temperature between the liquid crystal phase and an isotropic phase);
(3) having a low minimum temperature of the liquid crystal phase (a nematic phase, a smectic phase or the like), in particular, a low minimum temperature of the nematic phase;
(4) having an excellent compatibility with other liquid crystal compounds;
(5) having a large dielectric anisotropy; and
(6) having a large optical anisotropy.
If a composition containing the liquid crystal compound being chemically and physically stable as described in (1) is used for the display device, a voltage holding ratio can be increased.
Moreover, according to a composition containing the liquid crystal compound having the high clearing point or the low minimum temperature of the liquid crystal phase as described in (2) and (3), a temperature range of the nematic phase can be extended. Consequently, the compound can be used in a wide temperature range in the form of the display device.
In order to develop characteristics that are difficult to be output by a single compound, the liquid crystal compound is generally used in the form of a composition prepared by mixing the compound with a number of other liquid crystal compounds. Accordingly, the liquid crystal compound to be used for the display device is preferred to have a good compatibility with other liquid crystal compounds and so forth as described in (4).
In particular, the liquid crystal display device having a higher display performance in contrast, display capacity and response time characteristics and so forth has been required in these days. Furthermore, for a liquid crystal material to be used, a material having a low driving voltage, more specifically, a liquid crystal compound allowing to decrease threshold voltage, and a liquid crystal composition containing the liquid crystal compound and having a low driving voltage have been required.
As is well known, threshold voltage (Vth) is represented according to the following equation (H. J. Deuling et al., Mol. Cryst. Liq. Cryst., 27 (1975) 81):Vth=π(K/ε0Δε)1/2 wherein, in the above equation, K represents an elastic constant and ε0 represents a dielectric constant in vacuum. As is known from the equation, two ways are conceivable in order to decrease Vth: either increasing values of dielectric anisotropy (Δε) or decreasing K. However, according to the present technology, it is still difficult to control K as a practical matter. Therefore, under the present situation, a liquid crystal material having a large Δε is usually used to respond to the requirement. Under the circumstances, the liquid crystal compound having the large dielectric anisotropy as described in (5) has been actively developed.
Furthermore, in order to perform a good liquid crystal display, a thickness of a cell of the liquid crystal display device forming thereof and values of optical anisotropy (Δn) of the liquid crystal material to be used are preferred to be constant (E. Jakeman et al., Phys. Lett., 39A. 69 (1972)). Moreover, a response speed of the liquid crystal display device is inversely proportional to a square of the thickness of the cell to be used. Therefore, the liquid crystal compound having the large optical anisotropy has to be used for producing the liquid crystal display device being responsive at a high speed and also applicable to displaying moving images and so forth. Accordingly, the liquid crystal compound having the large optical anisotropy as described in (6) has been required.
So far, a variety of liquid crystal compounds having the large dielectric anisotropy and the large optical anisotropy have been prepared, and some of the liquid crystal compounds have been practically used. According to patent literatures No. 1 to No. 5, a compound having a —CF2O— bonding group, for example, three-ring compound (S-1) or four-ring compound (S-2) is disclosed. Because the compounds have a narrow temperature range of the liquid crystal phase and a low clearing point, a temperature range usable in the form of the display device is not sufficiently wide when the compounds are formed into the liquid crystal composition.
Furthermore, according to patent literatures No. 6 and No. 7, three-ring compound (S-3) having a —CH═CH—CF2O— bonding group is disclosed. The compound has the large dielectric anisotropy and the large optical anisotropy. However, the temperature range of the liquid crystal phase is not sufficiently wide.
